The present invention concerns a cylinder dryer for a paper machine or equivalent, consisting of a plurality of drying cylinders or equivalent rolls, between which the web and its lead strip is conducted, preferably supported by a fabric.
The main object of the invention is to provide a cylinder dryer of the kind above defined and wherein the web threading operation has been ensured, in particular when applying single-felt conduction or the like.
As known in prior art, a conventional rope carrier has been employed in association with single felt conduction. Since as a rule the rope grooves on the drying cylinders have a depth exceeding the thickness of the carrier ropes minus the felt thickness, the ropes and the felt edge will cross, whereby in the threading situation the edge strip shaped from the web may be broken up. Another drawback arising from the crossing of ropes and felt is the tendency of the ropes to fall off, which happens if the felt is carried towards the machine operator's side.
The web threading procedures of prior art based on the use of ropes, bands etc. are embarassed by a remarkable detriment owing to the falling off, damage or rupture of the rope, band or equivalent, or owing to failure of the equipment guiding these elements.
It has been suggested in certain threading procedures and apparatus of prior art to apply differential pressure (vacuum in the first place) to steady the web on the cylinders and to increase the evaporation during the run; and in this respect reference is made to the same applicant's earlier U.S. Pat. Nos. 4,183,148, 4,172,007 and 4,202,113 and to the U.S. Pat. No. 4,190,964 by J. M. Voith.
It is furthermore known in the art to utilize the so-called Coanda effect in the threading operation in the group interspace between the drying cylinders. But in order to serve its purpose, this procedure must detach the web and take it along, with the consequence that when applied within a group it leaves the web slack. When the aim is to achieve a fully closed conduction from the press section to the dryer, this advantage is lost if the end is threaded in the normal manner, because it becomes necessary to detach the web from the fabric supporting it.
Moreover, when the web is carried to one side, the web must move faster in order that the velocity component in the web direction might be constant, the transformation coefficient being 1/cos .alpha., where .alpha. is the deflection angle. As a rule, there is tension between the press and the first drying group, i.e., a positive differential speed in the direction of travel of the web, implying that on the draw in question the elongation E caused in the paper increases to be E/cos .alpha. as long as the threading process is in progress. This tends to favour web breaks. Moreover, when the web is carried to one side, other stresses are also produced in the web, increasing the risk of rupture of the lead strip.
It has also been suggested in the art to improve the web threading process by using a band that is conducted upon the web. This implies that extra equipment is used, and it is also accompanied by the detriments associated with the guiding and lateral moving of the band and with falling off and rupture, and with multiple folding. Reference is made in this respect to the U.S. Pat. No. 4,000,035 (J. M. Voith).
The web threading process usually requires manual operation, such as the use of blow tubes, and in which work steps a remarkable risk of accidents is present.
It is further known in the art to use subatmospheric pressure (vacuum) for keeping the web adherent in connection with a running mode as has been said. It is essential in these problem solutions known in the art, that in connection with the threading process on the surface of the cylinder/roll opening to the free space immediately in front of the end the subatmospheric pressure tends to discharge, whereby the end is detached from the felt, whereafter the suction in the region in question has no longer any substantial effect.
When the web that is being threaded has become detached, the stresses arising from air resistance and centrifugal force will also increase, impeding the threading operation, which may even fail totally. In this connection reference is made to the same applicant's Finnish patent application No. 793643.